Indium Phosphide Quantum Dots (InP QDs) are  promising non-toxic alternatives to the more established cadmium-based QDs for applications such as light-emitting diodes and display technologies. However, as-synthesized InP QDs exhibit low photoluminescent efficiencies and broad emission linewidths, hindering their successful implementation in commercial products. Those opto-electronic characteristics are strongly influenced by the surface chemistry of the QDs. Most importantly, surface defects such as under-coordinated atoms lead to localized trap states, strongly decreasing the emission efficiencies. To improve the properties of InP QDs, it is thus of uppermost importance to understand the termination of the inorganic QD cores, the binding modes of organic ligands, and to explore ways of tweaking the surface chemistry on demand.

The InP QDs studied in this work are synthesized via the aminophosphine-route, resulting in tetrahedrally shaped QDs, colloidally stabilized by oleylamine (OLA), and co-passivated by halide anions. In contrast to the traditional carboxylate-passivated InP QDs, the surface chemistry of the halide-amine InP QDs is still unexplored, and strategies to improve their emission qualities by surface treatments are limited.

To address this, we first performed ligand titration experiments with thiols and carboxylic acids to investigate the binding mode of OLA. Monitoring by quantitative nuclear magnetic resonance (NMR) spectroscopy and elemental analysis, we confirmed that OLA binds as a neutral L-type ligand, and can be replaced in an acid-base mediated ligand exchange. We further point out that surface-bound thiolates and carboxylates respectively decrease and increase the band edge emission of the InP QDs. Density functional theory (DFT) calculations confirm our finding, suggesting that thiolates easily form localized surface trap states.

In a second step, we added ZnCl₂ as Z-type ligand to  further increase the PLQY of the halide-amine co-passivated InP QDs. Most importantly, we find that pure metal chlorides strip OLA from the surface in a Z-type mediated L-type ligand replacement reaction, thus destabilizing the QDs. Interestingly, the complexation of ZnCl₂ with OLA prevents spurious aggregation. In this case, the binding of ZnCl₂ to the surface  triggers band edge luminescence. DFT calculations support these findings, and show how the adsorption of ZnCl₂ to under-coordinated P atoms can remove hole trap states.